The invention relates generally to electrostatic coating operations, and more particularly to instruments for measuring electrical parameters, including the resistance of liquid paint formulations and the surface resistance of articles to be coated electrostatically.
In electrostatic coating operations, the electrical resistance of paint and articles coated therewith must be within specified ranges to ensure electrostatic efficiency. The resistance of the paint is relatively high, and ranges generally between approximately 0.05 megohms and approximately 1.0 megohms, more or less, depending on the particular electrostatic process employed. If paint resistance is outside the desired range, the paint will have a tendency to draw electrostatic charge from the high voltage electrode of the spray gun, adversely affecting electrostatic efficiency. The resistance of articles, and more particularly the surface resistances thereof, normally considered to be electrically non-conductive, must be sufficiently conductive to accept electrostatically charged coatings. The surface resistance of articles ranges generally between approximately 500 Kohms and approximately 1.5 Gohms, more or less, depending on the particular electrostatic process employed.
Instruments for measuring the resistance of liquid paint formulations and the surface resistance of articles for electrostatic coating applications are known generally, and are used to determine whether additives or treatments are necessary to alter the resistance, or conductivity, thereof. Solvents, for example, may be added to paints to decrease conductivity, and the surfaces of articles may be treated with preparations, for example Ransprep.TM., to increase conductivity. It is also desirable to measure resistance generally, including the resistance of other liquids and articles, for applications besides electrostatic coating operations.
One known instrument for measuring the electrical resistance of liquid paint formulations applied in electrostatic coating operations is the Test Assembly.TM., Model No. 70408-00, available from ITW Ransburg Electrostatic Systems, Angola, Ind. The Test Assembly.TM. includes generally a plug-in paint probe having concentrically arranged first and second electrodes with a relatively low voltage supplied by the meter applied therebetween. The probe is immersed into liquid paint, and current flowing through the paint between the electrodes forms a basis for measuring and displaying resistance on an ohmic scale of an analog meter. The Test Assembly.TM. also measures current, and more particularly the short circuit current of electrostatic spray guns. To measure this current, a high voltage from an external source is applied to the spray gun when leads of the Test Assembly.TM. are connected between a high voltage electrode of the spray gun and a ground point, whereby the measured current is displayed on an ampere scale of the analog meter.
One known instrument for measuring the electrical surface resistance of articles coated electrostatically is the Sprayability Meter.TM., Model No. 8333-00, also available from ITW Ransburg Electrostatic Systems, Angola, Ind. The Sprayability Meter.TM. includes generally two probes, or electrodes, protruding from the instrument with a relatively low voltage supplied by the meter applied therebetween. The electrodes are touched firmly against a surface of the article to be measured, and current flowing through the article and between the electrodes forms a basis for measuring and displaying the sprayability of the article on a proprietary measurement scale of an analog meter. Other known meters for measuring the surface resistance of articles apply relatively high voltages through electrodes thereto, but the high voltages pose a shock hazard to personnel, and are undesirable.
The Test Assembly.TM. and the Sprayability Meter.TM. both include a battery powered vacuum tube. The vacuum tubes however are relatively sensitive, and must be calibrated prior to nearly every measurement to compensate for temperature variations and decreasing battery power. The vacuum tubes are also very fragile and easily broken if the instruments are not handled carefully. Moreover, the availability of replacement tubes is declining, and the cost thereof is increasing.
The vacuum tubes also provide a limited range of linearity for measuring resistance and current. Measurements are therefore most accurate over a relatively small range. To compensate for the limited range of accuracy, the Test Assembly.TM. includes a resistance scale select switch. Measurements however are most accurate only near the center of each selected measurement scale, and the meter is susceptible to damage during switching between scales. Since the surface resistance of articles and the resistance of liquid paints applied thereto in electrostatic coating operations varies over a relatively wide range, and since vacuum tube operated meters are accurate for measurement purposes over relatively narrow ranges, separate instruments are required to perform these various resistance and current measurements.
In both the Test Assembly.TM. and the Sprayability Meter.TM. a single battery applies a relatively low 45 volts generally to the corresponding electrodes for resistance measurements. This voltage level is certified to ASTM and ISO industry standards and ensures accurate resistance measurements within the ranges characteristic of most paints and materials used in electrostatic coating operations. The commercial availability of replacement 45 volt batteries however is limited and the cost thereof is high. Additionally, the 45 volt batteries are only available in carbon, rather than alkaline, form and therefore have a relatively short life span. Also, if the meters remain switched in the calibrate mode, as often occurs, the resulting calibration current will dissipate battery power quickly. In the Test Assembly.TM., if a test probe with paint lodged between the electrodes thereof remains plugged into the instrument the resulting current through the paint will dissipate battery power. These tendencies to deplete battery power are aggravated by the lack of an on/off switch on the Test Assembly.TM. and on the Sprayability Meter.TM.. The batteries must therefore be replaced frequently.
The present invention is drawn toward advancements in the art of electrical parameter measuring instruments useable for electrostatic coating operations.
It is an object of the invention to provide novel instruments and electrical circuits therefor for measuring resistance, especially the resistance of liquid paint formulations and the surface resistance of articles to be sprayed electrostatically, that are economical and that overcome problems in the prior art.
It is another object of the invention to provide novel resistance measuring instruments that operate efficiently on relatively low voltages supplied by standard, low cost and commercially available batteries, preferably a single 9 volt battery, the voltage of which is multiplied by a voltage multiplier circuit for applying a resistance measuring voltage between electrodes of the instrument.
It is a further object of the invention to provide novel resistance measuring instruments that comply with industry standards, particularly ASTM and ISO standards, applicable to measuring the resistance of liquid paint formulations and the surface resistance of articles to be coated electrostatically.
Another object of the invention is to provide novel electrical parameter measuring instruments that are accurate over relatively broad measurement ranges including resistance, voltage and current measurement ranges, and are thus suitable for measuring the surface resistance and resistance of liquid paints with a single meter.
It is a more particular object of the invention to provide novel resistance measuring instruments, useable for electrostatic coating applications, comprising generally first and second electrodes with a resistance measuring voltage applied therebetween, the resistance measuring voltage is generated by a voltage multiplier circuit having a first capacitor coupled to a first diode cathode and to a second diode anode, a second capacitor coupled to the second diode cathode and to an output diode anode, a first inverter coupled to the second capacitor. A periodic low voltage signal applied to the second capacitor is inverted relative to the same periodic low voltage signal applied to the first capacitor, whereby the periodic low voltage signal is multiplied to generate the resistance measuring voltage. Additional, similarly configured voltage multiplier stages multiply the periodic low voltage signal further.
It is another more particular object of the invention to provide novel resistance measuring instruments, useable for electrostatic coating applications, further comprising generally an A/D converter having differential inputs coupled to the first and second electrodes through a divider network, the A/D converter including an oscillator output for supplying the periodic low voltage signal to the multiplier circuit.
It is yet another more particular object of the invention to provide novel resistance measuring instruments, useable for electrostatic coating applications, further comprising a voltage measuring circuit switchable between the first and second electrodes and the differential inputs of the A/D converter for measuring voltages applied to the first and second electrodes, and a current measuring circuit switchable between the first and second electrodes and the differential inputs of the A/D converter for measuring currents applied to the first and second electrodes.
It is still another more particular object of the invention to provide novel electrical parameter measuring instruments, useable for electrostatic coating applications, further comprising a display coupled to and driven by the A/D converter, whereby resistance, voltage and current measurement results based on input signals to the differential inputs of the A/D converter are displayed visually.
These and other objects, aspects, features and advantages of the present invention will become more fully apparent upon careful consideration of the following Detailed Description of the Invention and the accompanying Drawings, which may be disproportionate for ease of understanding, wherein like structure and steps are referenced generally by corresponding numerals and indicators.